1. Field of the Invention
This invention relates to a wire saw for slicing workpieces, such as semiconductor ingots, into wafers and to a slicing method using the same.
2. Description of the Related Art
A wire saw comprises a row of wires arranged parallel to each other and at a fixed pitch. A workpiece is pressed against these wires running in parallel with one another in the directions thereof, while working fluid containing loose abrasive is supplied between the workpiece and the wires, thereby slicing the workpiece into wafers by a grinding action. In this way, a number of wafers having a fixed thickness can be obtained simultaneously. The working fluid is applied to the cut sections of the saw through nozzles as disclosed in Japanese Patent Laid-Open No. 61-125768.
For example, 6 hours is required to cut a silicon semiconductor ingot with a 5" diameter. It is necessary to keep the conditions constant so as to ensure high wafer-surface flatness precision.
However, due to the frictional heat generated between the wires and the workpiece and the frictional heat generated in the bearing sections of the rolling devices, the temperature of the rolling devices fluctuates during slicing, so that the pitch of the row of wires varies as a result of thermal expansion, resulting in the generation of waviness on the wafer surfaces formed by slicing. In view of recent developments in the miniaturization of elements of semiconductor integrated circuit, any waviness on the wafer surfaces will result in a substantial reduction in the yield of the semiconductor devices produced.
To mitigate this waviness, the temperature of the working fluid is kept constant. However, with the wire saw disclosed in Japanese Patent Laid-Open No. 61-125768, it is impossible to mitigate the fluctuation in temperature due to the frictional heat generated in the bearings of the rolling devices. In view of this, a rolling-device structure as shown in FIG. 7 has been proposed (Japanese Patent Laid-Open No. 62-251063).
This rolling device includes an axial core 1, a main oil passage 2 formed in the axial core 1, bearings 6-1 and 6-2 rotatably supporting the axial core 1, and lubricating-oil passages 3-1 and 3-2 formed between the bearings 6-1 and 6-2. Oil from an oil tank is supplied to an oil supply port 7 and flows through the bearing 6-1, the lubricating-oil passage 3-1, the main oil passage 2, the lubricating-oil passage 3-2 and the bearing 6-2, to be finally discharged through an oil discharge port 8. The temperature of the ail in the oil tank is maintained at a preset level.
However, when passing through the bearing 6-1, the oil absorbs the frictional heat generated at this bearing, with the result that the temperature of the oil increases. Since the oil whose temperature has been thus increased passes through the bearing 6-2, the heat absorption efficiency at the bearing 6-2 is generally different from that at the bearing 6-1. This difference also depends upon the temperature and the flow rate of the oil supplied to the oil supply port 7. Due to the difference in heat absorption, a temperature gradient is generated from one end of the axial core 1 to the other end, with the result that the coefficient of thermal expansion of the axial core 1 is not uniform along the longitudinal dimension thereof. As a result, the pitch of the wire grooves formed on a sleeve 9 is irregular, which is undesirable. Further, since the steel balls of the bearings 6-1 and 6-2 are closely arranged and are rotating at high speed, the flow-passage resistance at these bearings is relatively large. As a result, oil is apt to become stagnant at the bearing 6-1, so that the amount of heat absorbed by the oil at this bearing increases, thereby further enlarging the difference mentioned above.
Further, when the workpiece is a silicon semiconductor ingot, the length of the axial core 1 is, for example, as long as 1000 mm, resulting in a long flow passage and complicated structure. In addition, lengthening the main oil passage 2 may lead to eccentricity of the rotation center of the axial core 1, deteriorating the surface flatness precision of the wafer.
To ensure a long service life for the slicing wires, the sleeve 9 is made of synthetic resin, which is a heat insulating material, so that the slicing wires cannot be cooled by passing oil through the main oil passage 2.